ABSTRACT Primates, including humans, are expert at coordinating their arms and eyes in skillful behavior. Our goal is to understand the neural circuitry that underlies the combination of bimanual coordination and eye-hand coordination, which we call ?hand-eye-hand? (HEH) coordination. We are particularly interested in the early planning of bimanual movements, and the role-effector specific areas in the posterior parietal cortex play in that planning. We hypothesize that inter-areal and inter-hemispheric communication is necessary for HEH coordination. For example, the parietal reach region (PRR) controls primarily the contralateral arm. One way for one hand to know what the other is doing, so to speak, is for information to be exchanged between PRR in each hemisphere. The most direct pathway for such communication is through the corpus callosum, a major fiber tract connecting the two hemispheres. The relative accessibility of the corpus callosum provides an opportunity for causal tests of the role callosum plays in particular, and of inter-hemispheric communication in general, in HEH coordination. Lidocaine injections can reversibly block conduction through particular portions of the callosum, and behavior and neuronal activity can be compared in behaving animals before, during and after blockade. We predict that HEH coordination will be impaired when particular ?ber tracts within the callosum are blocked, and that there will be neuronal correlates of that impairment within the brain areas responsible for the behavior. Our ?rst Aim is to establish where in the callosum particular axonal tracts cross, and to verify that we can reversibly block conduction through those pathways. Next, for our second Aim, we will test speci?c hypotheses regarding which behaviors will be affected when particular pathways are blocked. We will consider pathways to and from the parietal reach region (PRR) and the lateral intraparietal area (LIP), an analogous area that codes saccade plans. Animals will perform interleaved, natural unimanual and bimanual reaches and saccades. We will then, in our third Aim, examine activity within PRR and LIP to determine how speci?c neuronal circuits are impacted by the transient loss of speci?c callosal pathways. Bimanual HEH coordination is critical for normal human behavior, yet the neuronal circuits involved remain largely unknown. This work will greatly expand our understanding of how parietal cortex achieves complex yet ?exible coordination of body parts. The information will be relevant to coordination in other effector systems, and will help us design the next generation of brain-computer interfacing prosthetics that can leverage natural coordination patterns and coordinate with existing limbs and eye movements. Further, we will learn fundamental facts about the role of the corpus callosum in the brain. Finally, this work will shed light on the general issue of long range communication across brain areas, and how this communication is related to brain function.